Ferromagnets refer to magnetic materials that become strongly magnetized in a magnetic field direction when exposed to an external strong magnetic field and retain their magnetism even when the external magnetic field is removed. In this case, individual atoms of the magnetic material behave like individual magnets.
That is, the ferromagnet is a material having magnet-like properties and typically includes iron, cobalt, nickel and their alloys. Individual atoms in the ferromagnetic material serve as individual magnets. The magnetic moments of these atoms are relatively less regularly aligned when they are not exposed to an external magnetic field, thus generally providing no magnet-like effects. However, when a magnet is brought close to the material, those atoms tend to align their magnetic moments with the external magnetic field and are therefore attracted to the magnet. Such regular alignment of atoms in a given direction under the influence of an external magnetic field is called “magnetization”, and the magnetized material per se can attract other ferromagnetic materials, as does a magnet.
As is known in the related art, the spins of electrons in the ferromagnet are aligned parallel to one another in the same direction, so magnetic moments responsible for magnetization form and increase. Further, magnetic domains are considered as large groups of atoms whose spins are aligned parallel with one another. Within the magnetic field, the magnetic domains whose spins are aligned with a magnetic field direction are produced or enlarged. Even after the externally applied magnetic field is removed, the spins of atoms are still aligned in the same direction for a long period of time, consequently resulting in appearance of remanent magnetization. When a temperature is elevated, thermal motion of atoms takes place in the ferromagnet, which breaks up regular arrangement of atom spins, thus becoming paramagnetic with loss of ferromagnetism. This temperature is called the Curie temperature above which a ferromagnetic material loses its ferromagnetism and becomes paramagnetic. This phenomenon is exploited in a variety of applications such as permanent magnets, magnetic permeability materials, and magnetostrictive materials.
Recently, IBM has proposed a new type of memory device, which consists of ferromagnetic nanowires and whose operation is based on the ferromagnetic domain wall displacement caused by the current injected into the magnetic nanowire.
FIG. 1 shows a conventional ferromagnetic nanowire-based memory device.
As FIG. 1 illustrates, magnetization directions of magnetic domains 110,120 whose magnetization directions are different from each other and are parallel to the nanowire surface are recorded in a conventional ferromagnetic nanowire 100, and the magnetic domains 110,120 are then moved by means of a current-induced domain wall displacement phenomenon where positional displacement of domain walls takes place upon application of an electric current to the nanowire.
As can be seen from FIG. 1, the conventional ferromagnetic nanowire 100 has a width (W) of from several to several hundreds of nm and a thickness (T) of from several to several hundreds of nm.
The information of the magnetic domains in the nanowire can be recorded and reproduced by recording and reproduction devices 130,140,150 positioned adjacent to the nanowire. Advantages of this technique are in that the positions of information recording and reproduction devices are fixed and the positions of information-containing magnetic domains can be electrically moved.
In spite of advantages such as a high recording density and non-volatility of information, conventional hard discs suffer from problems of impact susceptibility arising from a mechanically moving head of the device and also from high power consumption, which have been obstacles to the practical application of such hard discs to mobile storage devices.
On the other hand, flash memories, which are widely used as mobile storage devices, require very expensive production processes since one CMOS transistor should be inserted for each storage unit.
In contrast, the current-induced domain wall displacement-type memory device, to which the present invention pertains, is a storage device which is significantly advantageous as will be explained in the following. Specifically, the problems associated with high costs of flash memories can be solved because the memory device in accordance with the present invention requires only one CMOS transistor per nanowire where several tens to several hundreds of bits are stored. Further, as the mechanical rotational motion providing an important role in operation of the hard disc is replaced with the domain wall displacement which involves no mechanical motion, it is possible to achieve high impact resistance and low power consumption while retaining advantages of conventional hard discs, i.e. high storage density and non-volatility of recorded information.
In other words, the device of the present invention is significantly attractive as a memory device that provides prominent strengths of the hard disc (including high storage density and non-volatility of recorded information) simultaneously with high impact resistance and low power consumption, through the replacement of a mechanical element with an electrical element.
The most important factor in this technique is a current-induced domain wall displacement phenomenon, which was first theoretically proposed in 1980's by L. Berger and was recently experimentally observed by Yamaguchi, Klaui, Parkin and many other research groups.
According to the experimental research results reported up to date, a critical current density for the domain wall displacement using an electric current alone without the application of a magnetic field is about 108 A/cm2, which is 10 to 100 times greater than a value of 107 A/cm2 required for commercial application.
Increases in the critical current density disadvantageously result in very high power consumption for application of an electric current, and incapability to control the domain wall displacement due to generation of Joule heat.